Re: Weight vs. Fluid measurement (calculations)

[Guest guest]

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A little elaboration... A dry ounce (weight) is ALWAYS 28.34 grams.

A fluid ounce varies in weight based on a given material's specific

gravity.

If you have the MSDS for an oil, find the specific gravity listed

therein, and then you can calculate the fluid ounces, ml (or other

volume) per dry ounce (or other measure of weight).

Now, I know that the next question is " what is the formula for doing

this calculation? " but I am afraid I can't find it off hand, and the

website I usually use to do this work appears to have disappeared. I

am sure there are other sites with similar interactive tools, or

perhaps one of the more math literate members will rise to the

occasion?

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To convert from weight to volume (where W = weight in grams, V =

volume in ml, and d = specific gravity [density] in g/ml) --

V = W / d

To convert from volume to weight --

W = V x d

Or if you know the volume and weight and want the specific gravity --

d = W / V

Rather than memorize the formulas, it's easier (at least for old

folks like me, ha! ha!) to use " dimensional analysis " . Let's say you

have the volume, and want to know the weight that represents. So, you

know you want to end up with a " weight-unit " when all's said and done.

(ml) x (g/ml) = [(ml)(g)] / (ml) = (g) ... because (ml)/(ml) = 1

(remember that any value divided by itself is 1) and therefore those

units " cancel out " .

If you had divided instead: (ml) / (g/ml) => (ml)(ml) / (g) -- the

units did NOT cancel out, so this can't be the correct formula.

Likewise (g/ml) / (ml) => (g) / (ml)(ml) -- same problem.

Extending this to obtaining the volume, given the weight, the only

way to end up with a " volume unit " is to arrange things thus --

(g) / (g/ml) = (g)/(g) x [1 / (1/ml)] = ml

Like before, in the (g)/(g) part, the units cancel. Strangely enough,

the " reciprocal of a reciprocal " of a number is...the number! And the

same idea applies to units. So [1 / (1/ml)] is ml (!)

One thing you do have to be careful of is not to " mix " units. For

example, if you have a weight in ounces, but your specific gravity

(or density) value is in g/ml, you'll have to convert ounces to grams

or vice versa, to make the units consistent. Then you can apply the

formulas or do your dimensional analysis.

Another potential problem is that " not all drops are created equal " !

Drop size (the actual volume of a drop) depends on the density of the

liquid (which also depends on the temperature!), the diameter of the

dropper orifice, etc. That's why " final " formulas are most usefully

expressed in terms of weights. The next best thing would be to weigh

a certain number of drops (say 10 or 25) of the liquid in question --

then you could calculate a " grams/drop " density value for that

particular liquid, *with that particular dropper*. Using the drop

counts for each component in your mixture, you could then calculate

the weights of each component in your mixture.